Generally, sewage includes such pollutants as solids, organic matters, nitrogen and phosphorous. Sewage purification is processed in three steps based on the method of treatment: a primary treatment, a secondary treatment and a tertiary treatment. In the primary treatment, suspended solids that can be precipitated easily are removed physically. In the secondary treatment, which is a main sewage treatment step, dissolved organic matters and organic solids are removed. In this step, biological methods are used usually. In the tertiary treatment, organic matters and nutritive salts that have not removed in the secondary treatment are removed by using physical, chemical and biological treatment methods.
In the biological secondary treatment, one of the most popular methods is an activated sludge method, which aims for an organic removal efficiency of 90% in average. However, since the biological secondary treatment normally operating has a limitation in removing the pollutants of the wastewater, the tertiary treatment needs to be performed to produce water of a good quality.
In the tertiary sewage treatment, following methods are used based on the sorts of materials to be removed. If organic matters including suspended solids should be removed from sewage, such methods as rapid sand filtering, coagulating precipitation and membrane separation are used. If nitrogen of nutritive salts is to be removed, a biological method of nitrification and denitrification is used. If phosphorous should be removed, coagulating precipitation and other biological methods, a biological nitrogen and phosphorous removal method, and their modifications are widely used.
If rapid filtering method is used, the quality of the purified wastewater is relatively fine. However, this method has a problem in maintaining the sewage treatment facility. For example, backwashing should be frequently performed. In case of membrane separation method, it is not appropriate for large-size sewage disposal facilities, because it costs a great deal of money to construct and maintain the facilities. Besides, it requires an additional reaction tank and high technology to remove phosphorous and nitrogen and maintain the sewage treatment facility.